The new 20TB WD Red Pro is one of the largest NAS hard drives currently on the market, but at US$499 it might not be the most cost-efficient storage solution for owners of a network-attached storage server.
https://www.notebookcheck.net/Western-Digital-reveals-large-20TB-WD-Red-Pro-NAS-hard-drive-with-64GB-of-NAND-flash-memory.605347.0.html